1. Field of the Invention 7 The present invention relates to a control system for an automatic transmission.
2. Description of the Prior Art
Generally, an automatic transmission for an automotive vehicle is provided with a torque converter and a shift gear mechanism of which a plurality of frictional elements, such as clutches and brakes, is selectively actuated to switch a power transmitting path to thereby establish a desired shift stage automatically. The automatic transmission includes a hydraulic control unit for controlling a supply od hydraulic fluid to and from an actuator of the frictional element. The hydraulic control unit is provided with a regulator valve for adjusting a hydraulic pressure from an oil pump to a line pressure, a manual valve for manually switching a shift range, a plurality of shift valves for selectively actuating the frictional elements, and auxiliary valves actuated in connection with various operations. In recent years, the shift valve has been controlled through a solenoid so as to accomplish a sophisticated shift control in response to a vehicle operating condition.
In order to obviate a torque shock when the frictional element is engaged, there has been provided an accumulator on a hydraulic passage for supplying a hydraulic fluid as disclosed in Japanese Patent public disclosure (JP A2) No. 63-186055.
In the hydraulic control circuit disclosed the above Japanese publication, an accumulator is provided on a hydraulic passage communicated with a reverse clutch but not on a passage communicated with a low reverse brake. As a result, when a reverse clutch is engaged for establishing a R-range for reverse movement through a range select operation, a torque shock is obviated by virtue of the accumulator. However, when the low reverse brake is engaged for establishing a first stage in a L(low)-range or 1-range through a shift operation, a torque shock would not be able to be obviated.